On-Site X-ray Fluorescence Spectrometry Measurement Strategy for Assessing the Sulfonation to Improve Chemimechanical Pulping Processes

Minimizing the fiber property distribution would have the potential to improve the pulp properties and the process efficiency of chemimechanical pulp. To achieve this, it is essential to improve the level of knowledge of how evenly distributed the sulfonate concentration is between the individual chemimechanical pulp fibers. Due to the variation in quality between pulpwood and sawmill chips, as well as the on-chip screening method, it is difficult to develop an impregnation system that ensures the even distribution of sodium sulfite (Na2SO3) impregnation liquid. It is, therefore, crucial to measure the distribution of sulfonate groups within wood chips and fibers on a microscale. Typically, the degree of unevenness, i.e., the amount of fiber sulfonation and softening prior to defibration, is unknown on a microlevel due to excessively robust or complex processing methods. The degree of sulfonation at the fiber level can be determined by measuring the distribution of elemental sulfur and counterions of sulfonate groups, such as sodium or calcium. A miniaturized energy-dispersive X-ray fluorescence (ED-XRF) method has been developed to address this issue, enabling the analysis of sulfur distributions. It is effective enough to be applied to industrial laboratories for further development, i.e., improved image resolution and measurement time.


■ INTRODUCTION
The production of chemimechanical pulp (CMP/CTMP/ HTCTMP) has increased significantly in recent years for the production of hygiene products and even more so for packaging materials, such as paperboard and liner. In addition to the growing trend toward replacing plastics with renewable packaging materials that are easily recyclable and compostable, such as those produced by the paper industry, there is also an increased demand to improve the fundamental scientific understanding of pulp and paper manufacturing systems. As pulp is produced from chips, regardless of whether it is a highyield pulping process (chemimechanical or semichemical) or chemical pulping process (kraft or sulfite), the efficiency of impregnation is crucial. 1 It has always been very challenging to study the degree of effectiveness from the earlier suggested impregnation improvements. 2− 8 The chemithermomechanical pulp (CTMP) process, pretreatment of wood chips before defibration constitutes the core aspect of CTMP production. By increasing force in the direction of wood fibers during wood chipping, the wood structure is opened up, increasing the efficiency of CTMP impregnation. 9,10 The minimum amount of electric energy needed for the separation of wood into individual fibers is directly related to the softening of lignin. In CTMP, wood chips soften as a function of temperature in a preheater and in a refiner, where fiber separation is achieved. The wood chips at CTMP are steamed, followed by compression in a plug screw; the chips are allowed to expand in an impregnation liquor such as sodium sulfite (Na 2 SO 3 ) that lower the temperature than the steamed wood chips. 1,11−14 Based on the study, the softening temperature of black eastern spruce decreased by about 2°C when the sulfur content increased by 0.1% (as Na 2 SO 3 equivalents), while sulfur (as Na 2 SO 3 equivalents) content ranged from 0.3 to 2.8%. 15 In the CTMP process, the softening process is accomplished using sodium sulfite (Na 2 SO 3 ) solution at high temperatures (130−180°C) during preheating and refining. A key mechanism behind the sulfonation of wood is to reduce the number of cross-links in the lignin by breaking covalent bonds when sulfite ions react with α carbon in a coniferyl alcohol structure. 16 The structural change makes the wood softer at a certain temperature. 15,17,18 Earlier studies have examined the effects of sulfite concentration and temperature on wood sulfonation kinetics at pH 9. 19 Under CTMP conditions, sulfonation shifts fiber separation toward the middle lamella so that primary cell walls and middle lamellas are mainly separated. 15,20 It is because lignin is unevenly distributed within the fiber cell wall, and concentrations are higher in the middle lamella that a change in dynamic modulus will affect fracture mechanics during refining. 21 However, the combination of refining intensity and chemical softening by means of sulfonation and temperature-assisted softening during chip refining has already been examined with full-scale trials and implementations at the Holmen Braviken paper mill, Sweden. 22 The basic strategy of sulfonation for CTMP is to obtain the minimum possible shive content and well-separated fibers and thus linked to increasing the wood softening before defibration in a chip refiner. It has been studied that the feasibility of softening chips to produce CTMP that has well-preserved fibers with yields above 95%, shives content less than 1% before the screening, and energy consumption less than 200 kWh/h. 23 In the CTMP process, there is a key challenge in achieving even chemical distribution of sodium sulfite (Na 2 SO 3 ) and sodium hydroxide (NaOH) inside the wood chips, especially for the hardwoods due to the different sizes of the wood chips: length (∼20 mm), thickness (∼3 to 4 mm), and fiber width (20−40 μm) and length (1.5−5 mm) of Norway spruce. Previous scanning transmission electron microscopy-energydispersive spectrometry (STEM-EDS) studies on sulfonated wood chips of southern pines found variability in sulfur distributions in cell walls after the treatment of alkaline sodium sulfite. 24 During the impregnation process, the inner parts of the wood chips accept a much lower degree of sulfonation than the outer parts. A wood chip with less sulfonation or an unsulfonated chip tends to fracture in the outer secondary cell wall leaving a carbohydrate-rich fiber surface with different bonding characteristics. 25, 26 Increasing the sulfonation of the secondary wall increases the sheet density and strength, as well as the flexibility and conformability of fiber walls. 24 This uneven sulfonation is one of the factors that effects on pulp properties with higher shive content. By manipulating the degree of sulfonation of each wall layer, pulp properties could be controlled to meet specific end uses. However, to develop selective chemical impregnation methods for chemithermomechanical pulping (CTMP), fast and accurate measurements are therefore necessary. Due to the lack of fast measurements, it is currently difficult to optimize impregnation processes. Currently, there is no specific method for measuring the sulfonate distribution in wood chips and individual fibers at the microscale level. If there were, it would enable better knowledge of sulfonation before defibration, thereby perhaps making the manufacturing of CTMP and neutral sulfite semichemical (NSSC) more even and effective. However, the precise degree of unevenness, i.e., the degree of sulfonation and softening of each fiber in the chip refiner before being defibrated, remains largely unknown due to the complicated procedures used.
Although this sulfonation of CTMP research is of interest, few studies have been conducted earlier. Since the distribution of sulfur requires extensive measurements, it is costly to conduct in detail. In earlier studies, microtone cut layers of 100 μm wood chips were used to determine the sulfonation degree (sulfur content of the washed samples) and impregnation efficiency (total wavelength of the unwanted samples) of individual wood chips. 19,21,27 Sulfur concentrations were then determined via Schoniger combustion after dissolving wood chips in acid. Therefore, the presence of sulfur as sulfate (SO 4 2− ) ions was found whose content was determined using ion chromatography. 19,21 There was another CTMP study on birch wood blocks at various conditions. 12 In that study, the distribution of sulfur longitudinally was studied at various points using a scanning electron microscope (SEM) equipped with energy-dispersive X-rays. With such a method, sulfur and other counterion such as sodium concentrations appeared to be related to measurements of sulfur by ion chromatography and sodium by atomic absorption spectrometry. At both ends of the woodblock, the amount of sulfite (SO 3 2− ) was almost the same as expected, but it was reduced in the middle layer of wood. 12 Meanwhile, various analytical techniques can be employed to derive an elemental distribution map in different research disciplines. Some examples include X-ray absorption spectroscopy, X-ray fluorescence (XRF) spectroscopy, and scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS). 28−34 The research center Sensible Things that Communicate (STC) of Mid Sweden University, Sweden, has already been investigated at a laboratory-scale setup utilizing XRF on the coating of paperboard to simultaneously measure calcium (Ca) (3.7 keV) mapping and target copper (Cu) (8.0 keV) behind the paper. 33 By utilizing high-resolution measurements of sulfur distribution, we also anticipate that modern miniature X-ray-based techniques will contribute to the study of the sulfonation degree on-site impregnation process of CTMP.
This study suggests the use of a miniaturized, energydispersive X-ray fluorescence (ED-XRF) scan using a collimated X-ray source and utilizing a spectroscopy method based on energy-dispersive X-rays. Detection of light elements at miniaturized ED-XRF setups was challenging at first due to low fluorescence yields and air absorption from uneven wood chip surfaces. It was suggested that impregnated pulp samples or paper samples with a plain surface can be used instead. For this reason, we placed the paper sample produced from unbleached CTMP in a titanium (Ti) box in an atmosphere of air and helium (He). In a one-line measurement, the sample surface was scanned in a few steps, resulting in the production of an elemental distribution image of the substances as photon counts. Our study showed that it is possible to improve the setup by increasing the fluorescence yield for the study of the degree of sulfonation in chip refining of CTMP at an industrial lab scale. Our hypothesis is that the efficiency and evenness of fiber separation in the chip refiner are highly dependent on how evenly the chips have been sulfonated. We believe that a more even sulfonation distribution can significantly reduce the specific energy demand in chip refining for certain shive contents.
However, we are investigating and establishing the following precise on-site XRF methodology for validating whether the direct X-ray fluorescence method can be used to measure the distribution of the sulfonate group in CTMP paper samples ranging in size from a few millimeters (mm) to a micrometer (μm).

■ METHODS AND MATERIALS
Principle of XRF. X-ray fluorescence (XRF) is a technique for determining the elemental composition of a material by measuring the emission of characteristic fluorescent X-rays generated by a high-energy X-ray beam. By analyzing the photons emitted, elements can be determined to be present. 35 Instrumental Setup. In the XRF experiment, the sample was scanned to obtain the spectrum of a single-spot size, and then, the element of an interest peak was extracted and reconstructed by producing a spectrum of the element of interest. Using a collimated X-ray source and spectroscopic detector, this micro-X-ray technology can produce an elemental spectrum of sulfur and possibly other counterion such as sodium across wood chips or individual fibers in paper samples. An image of our XRF-based miniature setup is shown in Figure 1. To maintain the helium gas environment, a titanium cover palate was affixed to the titanium shield box. There was a 1.6 mm thick titanium plate with a 99.2% purity (metal basis). A two-dimensional (2D) stepper motor unit (Thorlabs) was connected to the sample box. To scan the sample in two dimensions, the stepper motor traveled a distance of 25 mm, and the minimum step size was 0.05 μm. An X-ray tube (Moxtek, 60 kV 12 W MAGPRO) and a spectrometer (Amptek X-123SDD) were mounted in air. The entire setup was covered with lead (Pb) sheets (thickness 1 mm, supplied by Nuclear Shields) into stainless-steel boxes to prevent radiation leakage.
The figure shows (a) a MOXTEK X-ray source, (b) an AMETEK spectrometer used to measure the spectrum of elements, (c) a 10 × 10 × 2.5 cm 3 titanium shield box to hold the sample at an helium gas atmosphere, (d) helium gas connection, and (e) a stepper motor 2D (XY directional movement).
The focal spot of an X-ray tube is typically 400 μm, and the beam diverges from its source. As a result, a large area of the sample was being used, and the fluorescence photons were accumulating in the spectrometer. To improve the XRF scanning image resolution, the X-ray was collimated using a pinhole to reduce the focus spot size. To obtain a scanning image of individual fibers, the beamline focal point needs to be as small as a few micrometers. An XRF setup was carried out with three-pinhole collimators. The two handmade pinholes had diameters of 10 and 50 μm made of tungsten carbide (WC, Alfa Aesar, 99.95% pure metal) that had shape defects in comparison to a commercial pinhole with a diameter of 100 μm made of gold (Au) and platinum (Pt). A pinhole reduces the beam intensity and increases the measurement time since only a portion of the X-ray passes through the collimator. Spotscan imaging was employed using two-dimensional precision translation stages, each with a 25 mm travel distance (Thorlabs).
Sample Preparation. Valmet's CTMP-712 pilot unbleached washed pulp was chosen for sulfur distribution to validate the XRF method. As the CTMP was thoroughly washed, there should be no other sulfur except for covalently bonded sulfur. The strategy was to estimate the differences in sulfur photon counts at high and low proportions of CTMP pulp from different handsheets. CTMP was diluted with bleached softwood kraft pulp (BSWK) of SCA reference kraft K44. An unbeaten low grammage handsheet of 20 gm/m 2 was measured for single line scanning in this setup. The low grammage handsheet 20 gm/m 2 was prepared by mixing CTMP and BSWK at different percentages according to the ISO 5269-2:2004 standard with tap water using a conventional sheet former with a surface area of 0.021 m 2 at the SCA R&D Centre in Sundsvall, Sweden. 36 For the study of sulfonation degree, four different proportions as 100% CTMP, 50% CTMP + 50% BSWK, 30% CTMP + 70% BSWK, and 20% CTMP + 80% BSWK were used.
Earlier studies reported a significantly lower sulfur and sodium circulation system in unbleached kraft pulp, and after bleaching, the total sulfur output and input were the same as those detected by the WinMOPS system in the Metso paper mill. 37 Since unbleached kraft (UBK) pulps with some lignin are thoroughly washed, bleached softwood kraft (BSWK) should not contain any residual sulfur unlike unbleached kraft pulps. It is also expected that the counterion sodium concentration would be too low to detect as well for kraft pulp. Therefore, bleached kraft pulp was mixed with unbleached CTMP pulp at different percentages to dilute the CTMP's sulfur content. However, fewer sulfur photons were expected gradually by decreasing the percentage of CTMP pulp consistency in these handsheets for the experiment.
Considering the measurement time, a higher concentration of sulfur and sodium was necessary to detect fluorescence photons from light elements. In contrast to sulfur, S (Kα 1 2.31 keV), sodium, Na (Kα 1 1.04 keV), was challenging to detect with the CTMP handsheet because of its low fluorescence yield. Seltin salt was used as an additional sample for the setup validation since it contains a high concentration of sodium and sulfur compared with other elements. Seltin salt, which is manufactured by Cederroth International AB, contains 50 g of NaCl (per 100 g) as well as KCl, MgSO 4 , and I. 38 ■ RESULTS AND DISCUSSION X-ray Attenuation. Upon excitation of the XRF, fluorescence photons must pass through the media and reach the spectrometer. Fluorescence X-ray radiation from the light element has relatively low energy (long wavelength), and it is severely attenuated by air. Before the XRF setup, it was simulated using a Monte Carlo-N-particle radiation transport code (MCNP). 39 Using X-ray-oriented programs (XOPs), photon transmission curves in air and helium gas relevant for light elements were plotted and can be seen in Figure 2. The simulation assumes 2 cm thickness of air or helium. For air, it results in a lowering of the sulfur peak at 2.31 keV and probably no sodium peak at all was left at 1.04 keV. Helium gas is therefore necessary for the detection of sodium, while sulfur can also be possible to detect in air although the signal decreases. It is possible to think of a vacuum atmosphere as an ideal situation in which there is no air absorption, but the difficulties associated with the maintenance of moving parts in a vacuum have major implications for the design of the instrument at a laboratory scale. For this application, a helium gas chamber is sufficient for measuring sodium.
In this study, a titanium box in a helium gas environment was designed to minimize the fluorescence photon absorption rather than a vacuum environment. Air environment was also considered here, but shield scatter photons in the air interfered with the measurements.
Validation of Feasibility of XRF Setup. To investigate the imaging system resolution of an XRF setup, it is necessary to choose the right pinhole for light elements with small sample sizes. In the air, a preliminary measurement of 60 μm of aluminum (Al) foil was performed. A sandwich structure is shown in Figure 3a, where the aluminum foil is sandwiched between copper and titanium plates. For primary verification of feasibility in the XRF setup, the 80 μm step size of one-line scanning measurement was used to determine the element distribution of Al, Cu, and Ti. A total of 30 steps were scanned. Using a 100 μm pinhole at an X-ray source, the measurement tube setting was 15 keV for 20 min for each step. The photon counts of the characteristic peaks of Al, Cu, and Ti in all measured spectra were integrated. The element distribution map for Al, Cu, and Ti can be seen in Figure 3b.
A fluorescence peak for aluminum appears at 1.48 keV (Kα 1 ) and 1.56 keV (Kβ 1 ). As the spectrometer has a 125 eV full width at half-maximum (FWHM), it cannot distinguish these aluminum peaks. Therefore, they merge into one peak in the output spectrum. Copper has a characteristic peak at 8.04  keV (Kα 1 ), and Ti has a peak at 4.51 keV (Kα 1 ), which is heavier than Al. A higher photon count corresponds to a higher concentration in the color bar. To draw elemental maps, we, therefore, combined spectral information with representative positions. As a result, the maps are in agreement with the sample structure. Due to the air absorption of the Al signal and the low fluorescence yield, the order of magnitude of photon counts from aluminum is much lower than those from titanium and copper.
To test the imaging system resolution of the XRF setup, the histogram of the aluminum signal with Gaussian fitting is shown in Figure 4. According to XRF images, for a 60 μm aluminum foil, the full width at half-maximum (FWHM) was 221.8 μm due to the pinhole size and magnification. Using an 80 μm scan step size, overlapped scanning was used. In scanning measurements, the spatial resolution of the XRF image is limited by the spot size of the source and the scanning step size. In this setup, a pinhole is used to collimate the size of the source beam. As a result of the geometry magnification and pinhole diameter, the size of the focused spot became relatively large for our application. The size of the focused spot can be reduced by reducing the pinhole to a sample distance while keeping the pinhole diameter, by introducing polycapillary optics or by scanning with overlap. A previous study reported the spatial resolution of the XRF image scanning pitch and the mode of scanning (stepwise or continuous) where the 50% overlap (the pitch size is half the focal spot size) is usually considered a possibility to improve spatial resolution. 40,41 Validation of Helium Gas in an XRF Setup. Following the investigation of X-ray attenuation and verification of feasibility of the XRF setup, it was further investigated on Seltin salt using a 10 μm pinhole at 8 keV fluorescence energy in a helium gas environment. In air and helium gas environment, a comparable peak of sodium (Kα 1 1.04 keV) and sulfur (Kα 1 2.31 keV) was observed; see Figure 5. In Seltin salt, the mass fraction for sodium was calculated to be 196.6 g/ kg. This is considered a high sodium concentration. However, it resulted in a detection rate of sodium fluorescence photons of only 28 counts in 17 h and thus indicated that the 10 μm pinhole greatly reduced beam intensity. As defined by the American Chemical Society (ACS), the limit of detection (LOD) is a concentration above SNR ≥ 3, which means the characteristic peak of an element at a given concentration exceeds the background by a statistically significant amount. We assume a linear relationship between the photon counts and the concentration in the XRF spectrum. Thus, the LOD for this setup was 29 g/kg in which a 10 μm pinhole was used.  According to earlier studies using a semiparametric neutron activation analysis, the sodium concentration in the bright paper was about 2 g/kg, which was lower than the LOD of this setup. 42 It is difficult to measure sodium in an XRF analysis because of its low concentration, low fluorescence yield, air absorption, and small pinhole. Moreover, we found that the Mα 1 -line from lead (Pb), interfering with the sulfur Kα 1 at 2.34 keV exists in the blank's measurement from lead (radiation shielding material outside the titanium box), and it interfered with the detection of sulfur in the Seltin salt. For the next real sample measurement of the CTMP and BSWK (50:50) handsheet, we shielded the X-ray tube with an aluminum pipe to avoid lead interference.
XRF Setup in Both Helium Gas and Air Environments. The sodium concentration for CTMP pulp samples was very low, making it difficult to detect the sodium peak with 10 and 50 μm pinholes. Therefore, we considered only measuring the sulfur from a 50 μm pinhole. As can be seen in Figure 6, handsheets of CTMP and BSWK (50:50) were investigated at 8 keV for 7 h in both helium gas and air environments. Our XRF setup clearly detected the sulfur peak (Kα 1 2.31 keV), especially when using the helium gas environment. Background noise around 2.3 keV was reduced by shielding the lead (Pb) Mα 1 -line signal with an aluminum pipe. Due to the small, irradiated area, low beam intensity, and low sodium concentration, the sodium peak was unclear in the spectrum. In addition, a polycapillary X-ray optic (not studied here) might be used as an alternative, which would provide a 150fold increase in X-ray flux gain over a laboratory-prepared pinhole. 43 Moreover, these considerations encouraged future measurements of sodium at a higher beam intensity in a vacuum environment at any synchrotron lab.
XRF Setup in the Air Environment. In spite of some background noise at this XRF setup, a single line scan of sulfur photon counts on CTMP handsheets with different percentages was performed in an air environment. The consideration of using in an air environment at laboratory setup was due to the consumption of helium gas that continues during use in the experiment that the balloon cylinder empties. Furthermore, the air absorption of sulfur is relatively smaller than that of sodium. An aluminum foil, however, was placed in front of the 50 μm pinhole to reduce the continuous X-rays from the tube. We scanned for 24 h at one spot point in single line scanning for 20 gm/m 2 handsheets produced from 100% CTMP, 50:50 CTMP/BSWK, 30:70 CTMP/BSWK, and 20:80 CTMP/ BSWK at 7 keV in an air environment. Figure 7 shows that the highest sulfur peak (Kα 1 2.31 keV) was observed for a 100% CTMP handsheet in comparison to 50−20% CTMP and BSWK mixtures. It should be noted that the background of the CTMP sample was corrected by a factor of three due to the  difference in thickness between these samples. As the sulfonate group is covalently bound to the lignin at CTMP, it can be easily detected by XRF.
At Figure 7, spectra resolved Mg (Kα 1 1.25 keV) from the X-ray tube, Al (Kα 1 1.48 keV) from the Al filter, S (Kα 1 2.31 keV) and Cl (Kα 1 2.62 keV) are accepted from the sample, Ar (Kα 1 2.96 keV) from the atmosphere, Ca (Kα 1 3.69 keV) from the paper sample, and Ti (Kα 1 4.51 keV) and Fe (Kα 1 6.40 keV) from the set housing materials. Fluorescence X-ray intensity is related to the thickness of a thin sample sheet. As a result, the thickest sample, 100% CTMP, exhibited a strong Ca peak. If different elements are analyzed in the same surroundings, then the saturation depth will decrease as the element's atomic number decreases. We do not correct for the difference in the thickness of the 100% CTMP sample compared to the other samples.
In XRF analysis, the area of a peak for a given element is directly proportional to its concentration within the sample volume. In Figure 8, the highest total counts of sulfur photons were observed at a 100% CTMP handsheet when the sulfur peak from all spectra was integrated and plotted. As the CTMP pulp consistency decreased at different handsheets, the number of sulfur photons decreased. As a result, the sulfur concentration variance was too small in 20 and 30% CTMP paper sheets.
Additionally, a small variance was observed in the 50, 30, and 20% CTMP samples from the scanning of four different handsheets for five steps (300 μm distance), as shown in Figure 9.
Because of the unequal distribution of sulfonate in the wood chips during the impregnation state, each step in single line photon counts will not be the same. Paper samples with CTMP consistency below 50% already had a less sulfonated fiber. It was difficult to determine the homogeneity of sulfur photon counts due to the uneven distribution of sulfur in each paper sample based on single-point scanning. It might be the case that the point we choose was a very low sulfonated fiber. The difference between 30:70 and 20:80 at Figures 8 and 9 could therefore not be distinguished from significant differences. Moreover, the single-spot analysis for 24 h took into account random errors such as generators and X-ray tube stability, as well as counting statistics. It was suggested for the next industrial lab setup that we continue single-fiber analysis with a higher beam intensity, aiming for 10 μm resolution. In this case, it is more effective to use polycapillary X-ray optics to encapsulate between 10 and 20 μm for single-fiber analysis rather than XRF pinholes. Polycapillary gain will provide 150  times more X-ray flux than our handmade 50 μm pinhole, resulting in a substantial reduction in measurement time. 44 ■ CONCLUSIONS XRF imaging using energy-dispersive X-ray spectroscopy (ED-XRF) has the ability to image light elements, especially sulfur, in both helium gas and air environments. Our lab setup for XRF imaging is validated by spectral measurement of Seltin salt for sodium and CTMP handsheets for sulfur individually. Our results show, however, that it is possible to develop a direct X-ray fluorescence method to measure sulfur in an air atmosphere. It also suggests the possibly to measure sodium, in particular at higher concentration levels, if an helium gas environment is used. Our setup has the capability to measure sulfur homogeneity with a spatial resolution in the order of 100 μm but must be further improved to measure homogeneity on individual fiber level. We suggest X-ray capillary optics as a way to improve the spatial resolution, as well as a way to shorten the necessary measurement time. In future studies, we suggest validation of an improved setup against sulfur imaging measurements from synchrotron facilities before going to large industrial lab trials. The manuscript was written through contributions of all academic and industrial authors. All authors have given approval to the final version of the manuscript.

Funding
This research was conducted in collaboration with FSCN (Fiber Science Communication Network) and STC (Sensitive Things that Communicate) at Mid Sweden University, Sweden, funded by the European Union regional fund, the Municipality of Sundsvall, the Kamprad Family Foundation within the EcoMat (Ecofriendly sustainable strong material) project, and finally Åforsk research grant within the micro-Xray project.